PTT, Dave Unwin
Keeping track of measurements and definitions in logbooks and elsewhere
While filling in my Jodel’s logbooks the other day I noticed that the airframe had just rolled over 1,000 hours, which is a bit of a milestone. By a cosmic coincidence, although neither my power nor glider logs appeared to be significant, when I added the two together for my total total time (as I do about twice a year) it was also a noteworthy number, and this got me thinking about the logbook and its origins. As the name suggests, originally the logbook was used to record readings from the ‘chip log’, which was for centuries the primary method of determining a ship’s speed through the water and absolutely fundamental to navigation. Basically, the chip log was a form of drogue (initially it really was just a log) that was connected to a thing called a log-line and dropped over the stern. The log-line featured regularly spaced knots, and while one sailor monitored a sandglass another counted the knots as they passed over the stern rail. It is a common misconception that these knots were 100ft apart, probably because an Imperial nautical mile, sometimes called an Admiralty mile, or more accurately, an Admiralty measured mile, is 6,080ft, and 100 feet times 60 minutes is 6,000ft. This would appear to make ‘one knot’ a hundred feet a minute, or one nautical mile an hour. In fact, most log-lines had the knots spaced at intervals of eight fathoms (48ft) and were used in conjunction with a half-minute sandglass,
It seems more likely that once the Dutch scientist Snell had calculated the world’s circumference as 24,024 miles in 1617, British mathematician Edmund Gunter proposed that using lines of latitude could be used to measure distance, and suggested that a nautical mile should be one minute (one sixtieth) of one degree of latitude. As one degree is 1/360th of a circle, one minute of arc is 1/21600 of a circle, making a nautical mile 6,080 feet, which is the length of one minute of arc at 48 degrees latitude (as the Earth is not a perfect sphere , a minute of latitude is not constant, and 48 was deemed a good compromise). Of course, as aviation has evolved we’ve grown used to a mish-mash of units, which is why although the length of the runway is in metres, the height of the trees that we’re trying to avoid at the far end is given in feet, and why an aircraft flying VFR in controlled airspace must remain 1,500m horizontally and 1,000ft vertically from cloud, and in a flight visibility of at least 5km at all times!
For many years western ASIS were often in mph and flight-planning was done using statute miles. There is an interesting albeit possibly apocryphal story that during WWII some RAF fighters were launched from a Royal Navy carrier to reinforce Malta, and that although the Navy had flight-planned using nautical miles, during the pre-launch briefing this wasn’t made clear and the RAF types just assumed the distance was in statute miles. On a 600nm flight the difference is not insignificant, and the last ninety miles would’ve certainly concentrated the mind!
Personally I prefer an ASI in knots, particularly when gliding as the variometer is also calibrated in knots, and with the altimeter in feet this allows you to make quite rapid final glide calculations. Not that anyone does any more of course, it’s all electronic - but it’s how I was taught. I’m not a fan of the metric altimeter (I find the scale expansion poor, as a single sweep of the dial is 3,280ft) but will allow that — occasionally the metric ASI has its uses — most memorably when I flew Peter Holloway’s magnificent Fieseler Storch several years ago. If you’ve ever stood next to a Storch you’ll know that — standing over three metres tall with a fourteen-metre wingspan and a MAUW of around 1,300kg — it’s quite a big machine. Consequently, when Peter said “Use eighty coming around the corner, and no less than seventy over the fence,” it sounded reasonable. If the ASI had been in knots, and he’d said “Use 43 coming around the corner and no less than 37 over the fence,” it would’ve sounded a lot less believable!
On a different tack...
It’s always been incumbent upon all aviators to keep their logbook correctly, and with the recent furore surrounding the well-known aviatrix Tracey Curtis-taylor now crossing over from internet forums to the mainstream media, I found myself wondering if the definition of ‘solo’ had somehow changed from the one I was familiar with. From what I can gather, Ms Curtis-taylor has gathered an impressive clutch of awards, medals and Honorary Doctorates while ‘replicating’ the solo flights of some amazing female aviators from the past in a beautiful Stearman, yet people persist in posting pictures of her arriving at various destinations with the front cockpit of said Stearman clearly occupied. As some of the more — shall we say ‘colourful’ — airmen of my acquaintance might ask, WTF is that all about? It’s certainly a subject that has piqued my interest.
As aviation has evolved we’ve grown used to a mish-mash of units I found myself wondering if the definition of ‘solo’ had changed...